The present invention relates generally to an underreamer to be used in a bottom hole assembly of a drillstring. More particularly, the present invention relates to a underreamer having retractable blades or pads configured to retract or engage a borehole along field-replaceable guide inserts or pins in a substantially linear path. More particularly still, the retraction or engagement of the blades or pads results from decreases or increases in working fluid pressure flowing through the retractable assembly.
Underreamers, in oilfield parlance, refer to downhole assemblies configured to enlarge existing boreholes. Underreamers function to enlarge smaller holes into larger-diameter boreholes. Often boreholes located below the lowest string of casing require bored diameters greater than the inner diameter of the next preceding string of casing. For these circumstances, an underreamer is installed behind a smaller drill bit and is run through the casing to the lower bore location. Once below the casing, the collapsible underreamer is expanded and a larger borehole is drilled. Once the larger bore is complete, the underreamer is retracted and the entire drilling assembly, bit, measurement equipment, and underreamer, is retrieved through the newly drilled borehole and casing thereabove.
Additionally, if the retractable cutters of an underreamer are substituted with retractable stabilizer pads, a retractable stabilizer can be effective in numerous subterranean drilling situations to centralize the drill string during operation. A retractable stabilizer can be employed, as above, to stabilize a retractable underreamer drilling assembly or, in the alternative, can serve as an adjustable gauge stabilizer. An adjustable gauge stabilizer is capable of reconfiguring its outer diameter to create an underreamed borehole of a desired size.
A recent exemplary expandable underreamer/stabilizer has been described in U.S. Pat. No. 6,732,817, issued on May 11, 2004 to Charles Dewey, et al., hereby incorporated by reference in its entirety. The invention disclosed in the Dewey patent relates to a three-bladed underreamer/stabilizer assembly wherein the three blades retract into and engage from a plurality of axial recesses having angled channels formed therein. The three blades of the Dewey patent engage the borehole by translating along the channels between a collapsed position and an expanded position in response to a differential pressure between an axial flowbore and the wellbore. The repetitive movement of the underreamer arms into and out of engagement in the presence of abrasive drilling fluids and cuttings can excessively wear the underreamer body thereby diminishing the useful life of the tool.
Unlike the prior art, the present invention reliably provides for direct movement of the blades into the expanded position resulting from the increase in pump pressure. Because the arms of the present invention are moved into engagement with the adjoining bore wall by direct movement of a piston or mandrel down the underreamer body, the circuitous hydraulic path of prior art tools, which can become clogged preventing free movement of the activator ring driving the arms into and out of engagement, is avoided. The present invention avoids this problem.
A hardfacing coating providing a low coefficient of friction of both the collapsible blades and the guides used to move these blades into and out of engagement with the bore wall additionally provides increased wear resistance and facilitates ready deployment under all well conditions. By utilizing a coating such as a QPQ nitride surface coating, the friction between the blades and guide inserts/pins is reduced. The hardfacing also makes the guide inserts/pins and blades more resistant the the abrasive drilling fluids present in a downhole environment. Because the guides and the blades can be replaced in the field when they become worn without the need to replace the entire underreamer body, the cost of using the underreamer with the present improvements is dramatically reduced over preexisting underreamer technology. The present invention constitutes a substantial improvement in the underreamer art by providing replaceable coated guides and blades.